1. Field of the Invention
The present invention relates to a method of controlling the rate of pulp and water flow in a multistage pulp bleaching system wherein each stage comprises a bleaching tower followed by a drum type washer and particularly to a method of controlling the rate of pulp and water flow out of a bleaching tower and over the following drum type washer in a bleaching stage and into any following bleaching stage such that the rate of pulp flow is known with accuracy and is more uniform than with conventional control methods.
2. Description of Prior Art
In a conventional system, control of the pulp entering the bleaching system is carried out utilizing a flow meter and a consistency controller at the entrance of the first stage. The pulp flow rate from each successive stage is then controlled by adjusting an outlet valve from each bleaching tower responsive to a level control instrument which measures the pressure in the tower as an indication of the level of contents in the tower. There are times, however, when it is advantageous to shut down one part of the plant for a short time and continue to operate the remaining stages by drawing down the level in the stage following the one that is temporarily shut down. Moreover, if there is a temporary upset in one of the latter stages then the previous stages can be operated for a short while by building level in the previous towers. The conventional control system is unsatisfactory for these cases where only part of the system is operative.
The washer following the first stage bleaching tower thickens the pulp composition to what is referred to as medium consistency, i.e., a consistency of about 10 to 16 percent pulp in water. The first stage is usually an upflow type of tower whereby the overflow is at the same rate as the input flow rate of the pulp and water mixture. The remaining two to five stages are usually run at the consistency coming off the previous washer drum except for the small volume of liquid from added chemicals and steam. The effluent from the first through the penultimate stages usually has a consistency of about 10 to 16 percent pulp in water and is run into a downflow type tower in a succeeding stage. The effluent from the last stage usually has a consistency of about 10 to 16 percent pulp in water. In some cases the pulp and water mixture is run through a smaller upflow tower before entering a larger downflow tower. In both these cases, however, the pulp and water mixture is held in the larger downflow tower for one to five hours by virtue of the tower size and rate of flow of the pulp and water mixture which is continuously introduced at the top and continuously removed at the bottom of such tower.
In most cases the pulp composition in the bottom portion of a downflow tower is diluted to about three percent pulp in water to facilitate pumping out of the tower and into the following washer and preferably is diluted further to one or one and a half percent pulp in water after leaving the tower and prior to entry to the washer to improve the pulp mat formation on the pulp washer and to give a better wash.
Some downflow towers are now equipped with medium consistency pumps for removal of the pulp composition from the bottom of the tower at the 10 to 16 percent consistency. This removal at medium consistency eliminates some of the problems experienced with introducing dilution water to a tower but creates a problem of instrument sensitivity since the pulp composition leaving the tower is then at the 10 to 16 percent consistency. A twelve percent consistency coming out of a tower instead of three percent increases the amount of pulp per foot of level measured by a factor of four and thus increases the flow rate errors caused by the larger amount of pulp represented by 0.5 percent instrument sensitivity in the level measuring device.
The system in most general use to inject water into the lower section of a downflow bleaching tower comprises nozzles located in a lower zone some two to three meters above the bottom of tne tower and spaced evenly around the tower. Agitation of the pulp composition is carried out in this lower zone with mechanical mixers to provide uniformity of consistency of the pulp composition in the lower zone. In essence then the 10 to 16 percent consistency pulp composition is floating above the three percent pulp composition in the tower bottom. This is possible since entrained air in the higher consistency pulp composition generally makes it have a lower specific gravity than the lower consistency pulp composition in said lower zone. The diluted pulp composition in the bottom section of the tower is pumped out as previously described and then additionally diluted to slightly over one percent pulp in water just before it enters the washer.
The flow rates of dilution water are not altered frequently in normal running and are not used for minute to minute tower level control purposes. In the normal control cycle when a downflow tower outlet valve is opened wider to lower the level in the tower, the flow rate of dilution water is not changed. The pulp and water in the agitated dilution zone in the bottom of the tower is then being pumped out faster with the same amount of dilution water being introduced. This increases the consistency of the pulp composition in the lower zone of the tower and additionally increases the rate of pulp flow out of the tower. This has a larger effect on uniformity than generally realized since at normal rates the contents in the bottom zone of a downflow tower is completely changed in less than five minutes.
The chemical additions to each stage are normally based upon a given percentage of the weight of pulp. For example, the third stage of bleaching is frequently a chlorine dioxide stage with a chemical application of about one percent chlorine dioxide based on the weight of the pulp which amounts to ten kilograms of chlorine dioxide per metric ton of pulp. Running at a rate of 750 metric tons of pulp per day (0.521 metric tons per minute), it is necessary to apply 5.21 kilograms of chlorine dioxide per minute to the pulp leaving the previous washer. The required percentage of chemical applied to the pulp is continually changing as is the pulp rate of production due to factors outside the bleaching operation. These chemical demand changes are known from other sensors within each stage and from data taken from other stages. When, however, the rate of pulp flow out of the towers varies due to the control function then the amount of chemicals applied is in error by a direct percentage of the tonnage rate error. In addition to these errors, the calculated chemical demand from other sensors and stages is also affected by erroneous tonnage values within the other stages. This leads to serious inefficiencies in the bleaching operation.